METEOR was proposed to compensate BLEU's and NIST's weaknesses [1]}, [2]}.
In short, METEOR was created to better correlate with human judgements by using more than word-to-word alignments between a hypothesis and some references.
The alignment is made according to three modules: the first stage uses exact match between word surface forms (Exact module), the second one compares word stems (Stems module) and the third one uses synonyms (Synonym module) from a lexical resource such as WordNet (available for English only in METEOR).
